Lousy Timelines

There is a good laugh to be had from this week's cover story in Newsweek about human evolution (here). Not only does it focus on the old news of evolution's fits and starts, but it begins with a reference to human lice that leaves out last week's breakthrough. It was in the New York Times (here). Body lice are usually more upsetting than interesting, but considered in the abstract, as bits of evidence, it turns out they provide some guidance on when certain crucial things happened along the human timeline. In particular one of the dates pushes some of the early speech pressures discussed on this blog back into Australopithecus times.

Lice are one of those things moderns don’t like to discuss or admit to having any experience with, although an informal query of people around me drew a number of confessions to having once, somewhere had a brief infestation. The pests themselves are small insects that inhabit the hairy parts of one’s body, crawling about and feeding on blood. Most mammals have hair covering their full bodies, so one species of louse is adapted to the whole animal. Chimpanzees and gorillas both have lice, each one serving as the host to a different species. Human head lice are closely related to chimpanzee lice, not surprisingly. When our ancestors went their separate ways, both sides of the family brought their parasites with them.

The lice on the human line encountered an unexpected problem. At some point the species began going bald over much of its body. Nina Jablonksi, an anthropologist at Penn State whose work has been discussed on this blog (here) has proposed that we lost our hair as part of the process of adapting to long-distance travel on the open grasslands of Africa. Hair decreased and sweat glands spread over their chest and back. She dates the change to early Homo, two to two-and-a-half million years ago. The lice study, however, suggests that date should go back about a million years, to 3.3 million years ago when somehow gorilla lice entered the human line where they became pubic lice.

As many a pet owner has learned, it is very common for parasites adapted for one species to hop on to another, so we do not have to insist on an entertaining, scandalous scenarios for the transfer of gorilla lice to our ancestors. But normally the alien infestation is short-lived. Gorilla lice on a chimpanzee, for example, would not be able to compete with the chimpanzee’s own lice who are already adapted to the chimp’s hair and blood. The survival of gorilla lice on humans suggests that there were already, 3.3 million years ago, islands of hairiness on a generally bare body. The descendants of the original lice were adapted to the head when an infestation of gorilla lice found an island around the privates where they could compete successfully and endure.

This blog has explored the relation between the decline in body hair and the rise of speech (see The Persistent Burden) and has suggested the following scenario: the loss of body hair made an infant more burdensome to the mother because infants could no longer cling to the mother’s hair. Mothers would have to carry the infant with them, either in their hands or in some kind of sling made from vines and leaves. This demand increased the risk that the infant would be mistreated or forgotten by a mother who must always be alert to the needs of her baby. As a counter to this increased risk of infant death babies began strengthening the emotional ties to their mothers by babbling—making meaningless, non-stressful sounds that enabled parent and child to share a happy moment together. This is the rise of emotional communication, a shared intimacy through meaningless sounds. Babbling is typically explained as an early step toward speaking, and these days it may be, but this scenario sees babbling as a trait with its own function. The dating of pubic lice to 3.3 million years ago puts nakedness and its consequent babbling deep into the Australopithecine epoch.

The third species of lice in the human line came much later and its importance was first reported some years ago (abstract here). This new louse arose as a metamorphosis of the head louse into a new body louse species that feeds on human blood but lives in the clothing. DNA analysis of the lice genomes dated the origins between 114,000 and 30,000 years ago. I see the Times article is now putting the date at the far end of that range, 107 thousand years ago, and is consistent with the picture that has been emerging for some time.

This new louse study suggests that each element of speech arose by itself, perhaps with a different species:

Australopithecus, 3.3 million years ago: babbling, rise of emotional communication.

Homo habilis, 2.2 million years ago: use of single words, start of meaningful communication.

Homo erectus, 1.8 million years ago: multi-words strings, draw attention to a topic.

Homo sapiens, 150 thousand years ago: sentences, with syntax.

Why one species per trait? We cannot be too rigorous with all these things and the links should not be taken as absolutes. But if the species and traits match up, why not use them in the story?